LibIPC+AudioServer: Allow unsolicited server-to-client IPC messages

Client-side connection objects must now provide both client and server
endpoint types. When a message is received from the server side, we try
to decode it using both endpoint types and then send it to the right
place for handling.

This now makes it possible for AudioServer to send unsolicited messages
to its clients. This opens up a ton of possibilities :^)

Libraries/LibAudio/AClientConnection.cpp

@@ -3,7 +3,7 @@
 #include <SharedBuffer.h>
 
 AClientConnection::AClientConnection()
-    : ConnectionNG("/tmp/asportal")
+    : ConnectionNG(*this, "/tmp/asportal")
 {
 }
 
@@ -76,3 +76,7 @@ int AClientConnection::get_playing_buffer()
 {
     return send_sync<AudioServer::GetPlayingBuffer>()->buffer_id();
 }
+
+void AClientConnection::handle(const AudioClient::FinishedPlayingBuffer&)
+{
+}